Gaseous discharge lamp starting circuits are well known in the art. Generally, gaseous discharge lamps require a higher voltage for ignition than for operation. This occurs because most cold gases act as insulators at ignition time, whereas during operation, the hot vapors become conductive. The high voltage requirements for ignition of discharge lamps can be accomplished by applying high open circuit voltages at the line frequency across the discharge lamp or employing starting circuits which provide high voltage pulses to generate initial discharge in the lamp.
A number of high-pressure sodium discharge lamp lighting circuits have been developed in recent years such as those disclosed in U.S. Pat. No. 4,072,878 to Engel et al. The Engel patent discloses an apparatus that provides for high-voltage pulses for starting a sodium discharge lamp by using the breakdown characteristics of a Zener diode to provide accurately timed starting pulses. The use of the Zener diode eliminates timing problems encountered when the breakdown characteristics of a glow lamp are used in somewhat similar fashion, as described in U.S. Pat. Nos. 3,917,976 and 3,963,958 to Nuckolls.
Another apparatus for starting and operating a high-pressure sodium lamp is disclosed in U.S. Pat. No. 4,143,304 to Hitchcock et al. The Hitchcock apparatus uses a voltage amplification circuit, utilizing two individual capacitors, the output of which is applied across the ballast reactor which is connected to the reactor in autotransformer relationship. Although the Hitchcock circuit works well, it does require a relatively large number of circuit components.
Further, various discharge lamp starting devices have been developed which employ non-linear dielectric elements. For example, U.S. Pat. Nos. 4,404,029 to Iwaya et al; 4,381,476 to Adachi et al; 4,399,390 to Oshita et al; 4,347,462 to Adachi and 4,513,227 to Labadini et al. The references show many variations in the use of starting circuits and devices which utilize the pulse generating capabilities of non-linear dielectric elements.
Further, in a co-pending application, Ser. No. 936,218 and assigned to the assignee of the present application, there is disclosed a starting and operating circuit for discharge lamps which employs a non-linear capacitor (NLC) in combination with a bipolar thyristor (SIDAC) and a voltage triggered switch. Therein, the inductive reactance ballast is a component of the starting circuit which generates the voltage pulses. The output voltage is limited by the inductance of the ballast. Thus, while the disclosed starting circuit provides improvement, the magnitude of the voltage pulse is increased by either increasing the inductance of the ballast or increasing the capacitance of the non-linear capacitor. Since the size of the inductance is essentially established, the non-linear conductor must be large enough to satisfy the starting requirements of the lamp with the possible resultant loss of desired small size and cost.
While such devices provide results in the areas intended, there still exists a need to provide a starting circuit for discharge lamps which employs inexpensive small components while providing a wide range of voltage pulses.
Accordingly, an object of the present invention is to provide an improved starting circuit for gaseous discharge lamps.
Still another object of the present invention is to provide a starting circuit which has a range of pulse voltages for initiating discharge which is simple and economical in construction and reliable in operation.